A blind rivet of this kind can be used not only for the grips which normally correspond to its shank length but also for smaller gripping thicknesses. The head of the drawing mandrel upsets the shank during the riveting operation, during which process the shank zone which projects from the parts to be connected and has a reduced wall thickness is deflected and forms a collar-shaped bead between the material surface and the closing head. Due to their wide range of use, blind rivets of this kind make unnecessary the production, acquisition and maintenance of stocks of a large selection of blind rivet sizes for all the gripping thicknesses which are required. Also, the risk of any confusion of the sizes occurring is considerably reduced.
A blind rivet of this kind which is known from obvious prior use consists of aluminium. Its shank has, approximately in the centre of its length, a zone of a reduced outside diameter and thus a reduced wall thickness. During its production, the shank of the blind rivet is pressed in its central zone between gripping dies. During this process, the aluminium rivet material is deflected on both sides of the die into the ungripped shank zones. This known blind rivet thus necessitates an elaborate production. Above all, the described deformation can only be brought about in aluminium rivets. It is not possible in steel rivets.
There is furthermore known a blind rivet (DE AS 1 099 276) which has sections of varying wall thickness. In the shank, which is cylindrical on the outside, there extends a cylindrical bore from the set head to approximately two thirds of the shank length. The remainder of the shank bore tapers conically towards the free shank end, as a result of which the wall thickness is also conically increased in this section. The drawing mandrel for this blind rivet has a half-round head whose diameter on its underside is between the shank diameter and the minimum hold diameter of the rivet. The edge of the head forms a cupped gripping point which is directed towards the shank end of the rivet. The drawing mandrel portion that directly adjoins the head is formed as a cone which increases away from the head. For the production of the riveted connection, this known rivet is deformed in two stages: At the start of the riveting operation, the conical zone of the drawing mandrel shank is outside the rivet shank bore. As soon as the thick end of this section is pulled into the rivet bore, it expands the section having the conical rivet shank bore in that it presses the shank wall to the outside in this section. When the head of the drawing mandrel comes into abutting contact with the rivet shank, the rivet shank has a bore which continues cylindrically at the same diameter and a section which conically widens on the outside towards the free rivet shank end. The second deformation is substantially effected in that the head of the drawing mandrel divides, with its cupped gripping point, the free shank end of the rivet shank into an external zone and an internal zone; the external zone, which is not covered by the drawing mandrel head, being pressed outwardly during this further pull-through for the closing head formation. The internal zone is pressed by the drawing mandrel head into the hollow space which is provided between the conical drawing mandrel section and the shank bore of the rivet. As the drawing mandrel is pulled through, the external zone of the rivet shank end flows around the head of the latter and largely envelops it. Following the breaking-away of the drawing mandrel at a predetermined breaking point, which is close to the thick end of the conical shank section that is close to the head, there comes about a largely tight riveted connection with a fixed drawing mandrel head and head shank. This known blind rivet has nothing in common with the species of the subject matter of the application since it is not intended for grips of varying thickness and the design of the drawing mandrel is different. The ratio of the drawing mandrel head diameter to the diameter of the rivet shank is in the case of the known blind rivet essential for the solution to the problem formulated in connection with this rivet, which solution is achieved by the described deformation of the blind rivet shank. An upsetting effect which with smaller gripping thicknesses might lead to the formation of the collar-shaped bead required therefor cannot be brought about in the case of the known blind rivet during the riveting operation.
Furthermore known is a blind rivet with sections of varying wall thickness (CB PS 860 123), wherein the shank bore has at the free shank end a section of reduced diameter and the shank has in this zone an outside diameter which conically increases towards the free end. This blind rivet is set by means of a pull-through mandrel which does not stay in the riveted joint. The pull-through mandrel has a shank with a conically widened end. As it passes through the rivet shank bore, the conical end of the pull-through mandrel presses the entire projecting zone of the blind rivet shank to the outside and, due to the external conical shape of this zone, brings it into gripping contact with the material to be connected. After the pull-through of the entire mandrel, the riveted connection has a bore of a continuous identical inside diameter. The pull-through mandrel of this configuration cannot exercise any upsetting effect at all. A hollow rivet connection only comes about if the shank length and the shank shape are exactly adapted to the intended gripping thickness. Should a closed riveted connection be desired, then a cylindrical pin has to be driven into the hollow rivet by hand. For the mentioned reasons, this rivet is not suitable for varying grips.
It is the object of the invention to provide a blind rivet of the above-mentioned type which can be easily produced from any material, especially steel, and which can nevertheless be used for a wide gripping range.